Underwater motive devices have been known since the 1950's. Most of those earlier devices were metal and were built like small submarines. Access was had through hatches which had to be securely bolted or clamped in order to resist taking on water at depth. As a result, the underwater motive devices were large, bulky and designed with a mind to limit outside access to limit the sealing areas provided for service access.
The early underwater motive devices were also expensive, and heavy such that the only effective market was professional divers because of both the cost and strength required to handle the unit under water. Because of the sealed nature of the units, rechargeablility caused considerable time in opening, inspecting and re-sealing the units.
Further, because early underwater motive devices were meant for serious under water work the full power of the unit was made instantly available in order to enable a sealed actuation switch to be provided through the wall of the unit to the outside. Instant-on full power was another reason that the early underwater motive devices were dangerous due to increased lack of controllability. The user had to be skillful to avoid being raked by nearby objects in addition to other user problems.
Another early problem was ballast. Most underwater motive devices were built for salt water density, but density can change from ocean to ocean (the Persian Gulf is saltier) and based upon water temperature. Adjusting the ballast was a problem because each time an adjustment was desired to be made, it involved a complicated breaching of the sealed outer housing.
What is needed is a underwater motive device which is (1) safer, (2) more easily ballast weighted, (3) more easily recharged and serviced, and (4) which is light weight and portable.